Metabolic disorders such as obesity and Type 2 Diabetes Mellitus (T2DM) are of major public health and economic concern in the United States and the world. Thus, understanding the mechanisms responsible for regulating metabolism and insulin sensitivity is of great importance. Within the central nervous system (CNS), the ventral medial hypothalamus (VMH) is vital for regulating whole body energy homeostasis. The cannabinoid 1 receptor (CB1R) has gained much interest in this area, as it is highly expressed throughout the CNS (including the VMH), and diet induced obesity (DIO) results in increased endogenous cannabinoid levels and CB1R expression. CB1R antagonist treatment in obese rodents and humans reduces adiposity and improves insulin sensitivity. However, numerous deleterious side effects are associated with these drugs and are believed to result from non-specific action on CB1Rs throughout the CNS. We propose to identify sites of CB1R expression within the CNS that are important for metabolism, and are associated with DIO and insulin resistance. Mouse studies indicate that the deletion of CB1R from the forebrain, including the VMH, confers the same protection against DIO and insulin resistance as total body CB1R deletions. We hypothesize that CB1Rs expressed in the VMH are key regulators of energy balance and contribute to the dysregulated energy homeostasis and impaired insulin sensitivity associated with obesity. We will determine if CB1R expression in the VMH is necessary and/or sufficient for the impaired energy homeostasis and insulin resistance associated with high fat feeding. To directly test our hypothesis, we have generated a unique line of mice that express Cre recombinase exclusively in steroidogenic factor 1 (Sf-1-Cre) cells. Notably, SF-1 neurons are expressed exclusively in the VMH, allowing us to selectively target this brain region using the Cre-loxP system. Sf-1-Cre mice will be crossed with mice expressing the Cbr1 allele flanked by loxP sites (Cb1rflox/flox), producing offspring (Cb1rflox/flox::Sf-1-Cre) that lack CB1R in SF-1 expressing neurons of the VMH. In parallel, we will generate mice in which CB1R are selectively expressed only in the SF-1 neurons of the VMH. Thus, we have generated a novel mouse model in which all CB1R expression is silenced by the presence of a floxed transcriptional blocker (Cb1rfloxTB/floxTB). In the presence of Cre-recombinase, the transcriptional blocker is removed and endogenous levels of CB1R are expressed. When Cb1rfloxTB/floxTB mice are crossed with Sf-1-Cre mice, their offspring (Cb1rfloxTB/floxTB::Sf-1-Cre) will express CB1Rs only in SF-1 neurons. Using these unique and complimentary mouse models, we will determine if CB1R expression in the VMH is necessary and/or sufficient for the regulation of energy homeostasis and insulin sensitivity. Specifically, we will examine the effects of a high-fat diet o energy homeostasis (caloric intake, energy expenditure, and body composition), insulin sensitivity (whole body and tissue-specific glucose disposal), and molecular signaling (markers of altered energy expenditure, insulin signaling, and potential mediators of insulin resistance) in our mouse models.